1. Field of the Invention
The present invention relates to an apparatus for and a method of polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to an apparatus for and a method of polishing a workpiece such as a semiconductor wafer which can control the amount of a material removed from a peripheral portion of the workpiece by a polishing action.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished to a flat mirror finish while the top ring and the turntable are rotating.
Attempts have heretofore been made to apply an elastic pad of polyurethane or the like to a workpiece holding surface of the top ring for uniformizing a pressing force applied from the top ring to the semiconductor wafer. If the pressing force applied from the top ring to the semiconductor wafer is uniformized, the semiconductor wafer is prevented from being excessively polished in a local area, and hence is planarized to a highly flat finish.
FIG. 15 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 15, the conventional polishing apparatus comprises a turntable 41 with an abrasive cloth 42 attached to an upper surface thereof, a top ring 45 for holding a semiconductor wafer 43 to press the semiconductor wafer 43 against the abrasive cloth 42, and an abrasive liquid supply nozzle 48 for supplying an abrasive liquid Q to the abrasive cloth 42. The top ring 45 is connected to a top ring shaft 49, and is provided with an elastic pad 47 of polyurethane or the like on its lower surface. The semiconductor wafer 43 is held by the top ring 45 in contact with the elastic pad 47. The top ring 45 also has a cylindrical retainer ring 46 on an outer circumferential edge thereof for retaining the semiconductor wafer 43 on the lower surface of the top ring 45. Specifically, the retainer ring 46 is fixed to the top ring 45, and has a lower end projecting downwardly from the lower surface of the top ring 45 for holding the semiconductor wafer 43 on the elastic pad 47 against removal from the top ring 45 under frictional engagement with the abrasive cloth 42 during a polishing process.
In operation, the semiconductor wafer 43 is held against the lower surface of the elastic pad 47 which is attached to the lower surface of the top ring 45. The semiconductor wafer 43 is then pressed against the abrasive cloth 42 on the turntable 41 by the top ring 45, and the turntable 41 and the top ring 45 are rotated independently of each other to move the abrasive cloth 42 and the semiconductor wafer 43 relatively to each other, thereby polishing the semiconductor wafer 43. The abrasive liquid Q comprises an alkaline solution containing an abrasive grain of fine particles suspended therein, for example. The semiconductor wafer 43 is polished by a composite action comprising a chemical polishing action of the alkaline solution and a mechanical polishing action of the abrasive grain.
FIG. 16 of the accompanying drawings shows in a fragmental cross-section the semiconductor wafer 43, the abrasive cloth 42, and the elastic pad 47. As shown in FIG. 16, the semiconductor wafer 43 has a peripheral portion which is a boundary between contact and noncontact with the abrasive cloth 42 and also is a boundary between contact and noncontact with the elastic pad 47. At the peripheral portion of the semiconductor wafer 43, the polishing pressure applied to the semiconductor wafer 43 by the abrasive cloth 42 and the elastic pad 47 is not uniform, thus the peripheral portion of the semiconductor wafer 43 is liable to be polished to an excessive degree. As a result, the peripheral edge of the semiconductor wafer 43 often are rounded during polishing.
FIG. 17 of the accompanying drawings illustrates the relationship between radial positions and polishing pressures calculated by the finite element method, and the relationship between radial positions and thicknesses of a surface layer, with respect to a 6-inch semiconductor wafer having a silicon oxide layer (SiO.sub.2) deposited thereon. In FIG. 17, blank dots represent calculated values of the polishing pressure (gf/cm.sup.2) as determined by the finite element method, and solid dots represent measured values of the thickness of the surface layer (.ANG.) after the semiconductor wafer was polished. The calculated values of the polishing pressure are irregular at a peripheral portion ranging from 70 mm to 74 mm on the semiconductor wafer, and the measured values of the thickness of the surface layer are correspondingly irregular at a peripheral portion ranging from 70 mm to 73.5 mm on the semiconductor wafer. As can be seen from the measured values of the thickness of the surface layer, the peripheral portion of the semiconductor wafer is excessively polished.
In order to prevent the peripheral portion of the semiconductor wafer from being excessively polished, there has been proposed a polishing apparatus having a retainer ring comprising a weight which is vertically movable with respect to a top ring as disclosed in Japanese laid-open patent publication No. 55-157473. In this polishing apparatus, the retainer ring is provided around the top ring and pressed against an abrasive cloth due to gravity.
The top ring of the above proposed polishing apparatus is capable of varying the pressing force for pressing the semiconductor wafer against the abrasive cloth depending on the type of the semiconductor wafer and the polishing conditions. However, since the retainer ring cannot vary its pressing force applied against the abrasive cloth, the pressing force applied by the retainer ring may be too large or too small compared to the adjusted pressing force imposed by the top ring. As a consequence, the peripheral portion of the semiconductor wafer may be polished excessively or insufficiently.
According to another proposed polishing apparatus disclosed in Japanese patent publication No. 58-10193, a spring is interposed between a top ring and a retainer ring for resiliently pressing the retainer ring against an abrasive cloth.
The spring-loaded retainer ring exerts a pressing force which is not adjustable because the pressing force is dependent on the spring that is used. Therefore, whereas the top ring can vary its pressing force for pressing the semiconductor wafer against the abrasive cloth depending on the type of the semiconductor wafer and the polishing conditions, the pressing force applied to the abrasive cloth by the retainer ring cannot be adjusted. Consequently, the pressing force applied by the retainer ring may be too large or too small compared to the adjusted pressing force imposed by the top ring. The peripheral portion of the semiconductor wafer may thus be polished excessively or insufficiently.